CVTs are increasingly used in vehicles and other applications. The present invention is concerned with traction drive CVTs. Two major types of traction drives are used. One uses a belt running between adjustable pulley sheaves. The other, a toroidal variator, uses a roller that runs between two discs with the negative shape of a toroid machined into it.
There are two types of toroidal variator, a full toroidal variator and a half toroidal variator. These variators include discs that are clamped onto the rollers with high forces allowing the rolling contacts to transfer force using friction or traction as it is otherwise called. The present invention is concerned with full toroidal variators, and more particularly with double roller full toroidal variators, or DFTVs. In this class of device, a pair of rollers runs on two discs and also on themselves. An example is described in patent application No. PCT/AU2010/001331 by the present applicant, the disclosure of which is hereby incorporated by reference.
The DFTV disclosed therein uses two rollers, each of which is formed with both a conical surface and a toroidal surface. The toroidal surface of each roller rolls on the discs while the conical surfaces roll on each other. The clamping force from the discs is arranged so that it passes directly from one toroidal contact to the other contact and through the line where the conical contact between the rollers exists so that the two rollers are forced into a position relative to each other by the relative angles of the cones. Typically the conical angle is equal so that there is no relatively sliding of the rollers along this line caused by any force component imbalance.
Prior art FIG. 1 shows a cross section through such a device where the rollers 1 are clamped between an input disc 2 with an inner surface 23 and an output disc 3 with an inner surface 22. The discs 2, 3 internally define a toroidal space. Each roller 1 has a toroidal surface 21, i.e. shaped to engage the surfaces 22, 23 of the toroidal surface, as well as a conical surface 20 for engaging the other roller in the pair. The clamping force is resisted by rollers 1 along the reaction line 11. The rollers 1 are supported on axles 8 that are captured in yokes 4, which are in turn attached to trunnions 5. The clamping force is resisted along the line 11 that joins the two points of contact of each roller with each disc. The rollers are connected to trunnions 5 with a form of typically flexible connection allowing one or more degrees of freedom of movement but in a way that allows tangential forces to be transferred from one disc to the other.
Patent application PCT/AU2012/000476 by the present applicant, the disclosure of which is hereby incorporated by reference, describes a method of controlling roller position that can be applied to both double roller and single roller designs.
This application discloses an arrangement in which the rollers are supported in yokes that are mounted on a trunnion, and the trunnion can be rotated about an axis that is tangential to the circle formed by the centre of the toroidal cavity. The rotation of the trunnion causes the rollers in the double roller format to “steer” relative to each other so that the conical contact moves slightly off line relative to each roller. That roller then rolls in a direction along the disc surface that causes a ratio change in the variator requiring very little force to execute.
Prior art FIG. 2 depicts such an arrangement in which the rollers 1 are supported on axle bearings 9 that roll on axles 8 that are fixed into yokes 4 that are connected to a trunnion 5 by a ball joint 6. The trunnions 5 can rotate on an axis 5a that generally lies along a line in a plane that is perpendicular to the axis of rotation 24 and that passes through the centre of the toroid 26, and is tangential to the circle that forms the centre of the toroidal cavity. The rollers in their undisplaced position roll on each other on the conical contact line 10.
Yokes 4 are free to rotate on the ball joints (not visible in this view) in any direction, one of these degrees of freedom allowing the rollers 1 to steer when the trunnion 5 is rotated. The line of force 11 is aligned symmetrically and normal to the clamping force.
The steering allows each roller 1 to roll across the toroidal surface 21, 23 and adopt a different ratio position. In their normal rotating state the axes of each of the rollers pass through the axis of rotation of the discs with all three axes lying in the same plane.
It is an object of the present invention to improve the efficiency of a double roller full toroidal variator.